An image on a television screen consists of pixels, arranged horizontally in rows, generally offset vertically by one pixel position from one another. Each pixel is assigned three values which indicate the respective intensities of the red, green, and blue components of the pixel. A video image is generated by sequentially displaying the rows of pixels as horizontal lines of the image.
Existing analog broadcast standards such as NTSC, PAL and SECAM use two video fields to generate a single video frame. Each field includes one-half of the horizontal lines that make-up the image frame. One field includes all of the odd numbered lines of the frame and the other field includes all of the even numbered lines. One way to eliminate artifacts like image flickering is to convert the interface-scan field into progressive-scan frames. In a progressive-scan frame, both the odd and even image lines are displayed.
One way to generate progressive-scan frames from interlace-scan fields is to interpolate interstitial lines in each field. Thus, the lines of the odd field are used to interpolate even-numbered lines and the lines of the even field are used to interpolate odd-numbered lines.
U.S. Pat. No. 7,362,378 B2 discloses a method for converting an interlaced scan image, having a plurality of pixels arranged in a matrix of rows and columns, to a progressive scan image processes interpolate picture elements at target pixel positions between two successive rows of the interlaced scan image. The method determines if a pixel on a current row is an edge pixel and if the pixel is an edge pixel, it determines an approximate angle for the edge based on a difference between the column subset of at least the edge pixels on the current row and the column subset of at least one edge pixel on a previous row. The method uses the angle to determine which pixel on the current row and previous row correspond to the target pixel and interpolates the value of the target pixel from the corresponding pixels.
Thus, today's “interlace-to-progressive” signal processing methods use techniques to first identify an edge and/or slanted line and second interpolate the missing information along those edges and/or slanted lines. These two basic steps require usually a lot of logic to clearly detect and/or identify an edge and/or slanted line. This additional logic is mostly not suitable to be implemented in either software and/or hardware due to the non-linearity of the decisions to be made.
The disadvantage of current state of the art interlace-to-progressive algorithms is that they achieve poor quality on lines and edges which are slanted towards the horizontal axis. This is typically known as “stair case-effect”. This artifact disturbs the visual impression of the picture significantly.